In order to study the characteristics of magneto-optical imaging of laser welding cracks under rotating magnetic field excitation in frequency domain, the method of 2-D discrete Fourier transform for magneto-optical image of laser welding crack was adopted. Theoretical analysis and experimental verification were carried out. Spectrum data of laser welding crack under different excitation intensities of rotating magnetic field were obtained. Combining spatial characteristics of crack magneto-optic maps, statistical analysis was carried out on the points whose gray value of crack spectrum was 255. The results show that, frequency domain characteristics of magneto-optic image of laser welding crack have a certain corresponding relationship with spatial domain characteristics. During a period of change (885 frames of magneto-optic map), the corresponding spectrum map will be a process of first decreasing, then enlarging, then decreasing, then enlarging or the opposite, and finally returning to the initial state. The results verify the correctness of magneto-optical imaging law of cracks under rotating magnetic field. It is helpful for non-destructive detection of laser welding defects.